Conventionally, heat treatments such as quenching, tempering, annealing, and a solution heat treatment are used as heat treatments for a steel tube or pipe. Those heat treatments are being performed selectively in accordance with a purpose such as performance required by customers or homogenization of product quality.
Generally, the heat treatments for a seamless steel tube or pipe are performed online from the viewpoint of productivity. For example, in Japanese Unexamined Patent Application Publication No. 2002-30342, a heat treatment equipment line in which a heating furnace for quenching, equipment for quenching, and a tempering furnace are effectively arranged is proposed to enhance efficiency and compactness.
Nowadays, on the other hand, the environment of usage of seamless steel tubes or pipes for Oil Country Tubular Goods, which are used in oil wells and gas wells for crude oil and natural gas, is becoming harsher than ever, the tubes or pipes are required to have not only high strength but also high performance including high corrosion resistance. In view of such a situation, for example, a high strength stainless steel tube or pipe for Oil Country Tubular Goods containing 15.5% (mass %, simply represented by % hereinafter) or more of Cr and having a strength higher than 654 MPa (95 ksi) in terms of yield strength, excellent CO2 corrosion resistance, and high toughness, which is disclosed in Japanese Unexamined Patent Application Publication No. 2005-336595, has been developed and used.
However, in a high strength stainless steel tube or pipe containing a large amount of Cr as described above and which contains alloy chemical elements such as Ni and Mo, the martensite transformation finish temperature (Mf point) is about room temperature or equal to or lower than room temperature (25° C.). When a quenching and tempering treatment is performed on that high strength stainless steel tube or pipe using conventional heat treatment equipment, since a cooling stop temperature after quenching varies due to a change in room temperature and constraints because of processes in a continuous operation, there is a variation in the volume fraction of a residual austenite phase before tempering is performed. Therefore, there is a problem in that, since mechanical properties such as strength and toughness become unstable after a heat treatment has been performed, these mechanical properties vary among products.
It could therefore be helpful to provide a heat treatment equipment line for a seamless steel tube or pipe, and a method of manufacturing a high strength stainless steel tube or pipe with which stable product quality can be obtained after a heat treatment has been performed.